The field of the invention is medical imaging and particularly the acquisition of x-ray images for use in image-guided medical procedures.
Stroke afflicts more than 700,000 people yearly in the United States alone. Approximately 85 percent of strokes are caused by an ischemic mechanism, either from local thrombosis or clot embolism. The only FDA-approved method for clinical treatment of acute ischemic stroke is a thrombolytic agent, tissue plasminogen activator (tPA), started intravenously within 3 hours of stroke onset. Patients are selected for tPA treatment based on clinical features and basic non-contrast CT (computed tomography) imaging criteria, with the exact site of clot not directly confirmed in most instances.
Following the clinical trials which lead to tPA approval, subsequent real-world use in many centers has confirmed improved outcomes in patients treated with tPA. However, initial enthusiasm for thrombolysis has declined to more cautious and limited deployment in most centers. Limitations of IV injected tPA include the short time window available to identify and treat patients, controversies on how to select candidates for treatment, the relatively modest effectiveness and outcome benefit perceived by clinicians, and the rare but often fatal hemorrhages which occur as a complication. More than five years after introduction of the drug, fewer than 2 percent of acute stroke patients are treated with tPA.
Recent trials have demonstrated that the 3 hour time window in which the benefits of tPA outweigh its risks can be lengthened. During a 2 hour period, pro-urokinase was infused and angiographic images were acquired to monitor clot lysis and assess blood flow. Even as late as 6 hours after stroke symptom onset, high rates of clot lysis can be achieved. Clinical outcome in these cases often is determined by the residual perfusion of affected tissues rather than the state of clot lysis as seen in conventional angiograms. Thus, the acquisition of perfusion images of the affected tissues is an important tool in assessing the advisability of using tPA treatment.
Contrast techniques can be used with multi-slice x-ray CT to provide perfusion parameter maps. Such perfusion images are acquired after the injection of a contrast agent using an x-ray CT system. The acquisition of such images and the assessment of brain perfusion and its relationship to infarcted areas has become central to advanced clinical techniques and the making of decisions regarding patient selection for acute treatments. Perfusion assessment brings an individual, patient-specific physiologic-based method (not just time or anatomy) to select candidates who are most likely to benefit, while simultaneously excluding patients who could be hurt by these potentially risky treatments.
While it has become evident that perfusion measurement is important in acute stroke, none of the current imaging methods can be performed directly in the angiographic suite where intra-arterial thrombolysis is conducted. MR and CT-based perfusion methods have shown value, but must be done before or after the interventional angiographic procedure itself. Other traditional perfusion techniques using PET, SPECT, or XeCT have also been used for acute stroke, but are more lengthy procedures, have not seen widespread practical utility, and cannot be performed during the angiographic procedure.
Some medical centers have combined angiography-MRI facilities (“XMR”) to offer interventional angiography-perfusion/diffusion capability, but these facilities still require moving an acutely ill patient from one imaging room to another to conduct interventional angiography and perfusion assessment in an interleaved fashion. All these current approaches are inefficient and require moving a patient who may be at risk of bleeding due to drugs and indwelling intra-arterial catheters.